Heat-resistant ferritic alloy



Patented Dec. 25,1951

HEAT-RESISTANT FERRITIC ALLOY Erik H. Mattiasson Hiigglund, Southport, Conn., and Nils Giista Rehnqvist, Hallstahammar, Sweden, assignors to Aktiebolaget Kanthal, Hallstahammar, Sweden, a corporation of Sweden No Drawing. Application April 6, 1950, Serial No. 154,418. In Sweden March 10, 1945 1 Claim. -(o1. 75-4-26) Our invention relates to a heat resistant, ferritic alloy usable as electric resistance material for high temperatures as well as for manufacturing details of construction exposed to high temperatures.

This application is a continuation-in-part of our copending application Ser. No. 610,206, filed August 10, 1945, now abandoned.

As heat resisting alloys for the above named kind two main types have been used hitherto.

One alloy type consists essentially of nickel and chromium. These alloys are in character austenitic and have good mechanical strength, if they are heated to high temperatures also after long use. The heat resistance of these alloys is, however, limited and moreover, they are attack} able by sulphur and sulphur compounds. j

The other main type of heat'resistant alloys consists of such in character ferritic; these alloys consist of iron and chromium, possibly with an addition of aluminium, cobalt and nickel. They contain moreover small amounts of silicon, manganese and other materials normally occuring in steel in form of impurities. The heat resistance of the type containing cobalt is particularly far surpassing the austenitic alloys. However, the ferritic alloy shows the trouble being disposed for strong brittleness at room temperature after long heating at high temperatures.

Our experiments have shown that by an addition of extremely fine particles to theabove named alloys of ferritic type which not even by heating to high temperatures are dissolved in the alloy in noteworthy degree, the named disposition for brittleness may be diminished or wholly prevented. The condition for this is, however, that the. particles occur in ,a large number and that they are uniformly distributed in the whole ferritic basic composition of the alloy; Those particles consist of chemical compounds with high heat resistance and especially of oxides F or carbides or oxides and carbides.

The addition of the particles may occur either to the alloys being in melted condition or the particles may be mixed in the alloys in the powdermetallurgic way, if the alloy is prepared according to this method. In both cases the particles may be added ready-prepared in extremely fine divided form.

If the alloy is prepared by the melt process the particles may be obtained also in separated form in the basic composition of the alloy by adding the basic elements composing the particles to the melt in convenient proportioned amount and in stoichiometric relation to each other.

' If the particles consist of oxides, one or more strong oxide forming metals and suitable proportioned amount of oxygen may be added to the melt whereby the reaction product is obtained in the form of fine oxides uniformly divided in the alloy.

The increase of the oxygen content of the melt may be performed either by using a slag rich in oxide, by blowing in oxygen, addition of oxygen delivering products for by giving the oxygen containing atmosphere surrounding the melt higher pressure.

Both methods, that in adding finished compounds to the melt or forming the compounds in the melt, may be combined with each other.

As oxide forming metals the following metals are especially suitable:

Li, Be, Mg, Ca, V, Sr, Zr, Ba, Ce, Ta, Ti,- Th and a number of other metals known as strong oxide forming metals.

As carbide forming metals the following metals are especially suitable:

V, Zr, Nb, Mo, Hf, Ta, W, Ti, Th.

Several of the above nam d me a have, if solved in the basic composition of the alloy, a deteriorating influence on the heat resistance of the alloy. Therefore, it is necessary that so much carbon or oxygen or both are present in the alloy that the whole amount of the added metal is bound in the form of oxides or carbides or oxides and carbides.

' For preventing an oxidation of the aluminium 'by forming of oxides in the meltit is preferable to add the necessary amount of the aluminium after forming the oxides in the melt by adding suitably proportioned amounts of oxygen and strong oxide forming metals.

For example a slag rich in oxygen is added to the melt and then the necessary amount of one or more strong oxide forming metals. After the necessary amount ;of oxide is formed in the melt the required amount of aluminium may be added.

In the process of forming carbides in the melt it is preferable for securing the best results to reduce the carbon content of the melt as much as possible for example to ODS-0.08%, to add the required amount of carbon for example in the form of a carbon containing material as pig iron containing 34% C. and then the necessary amount of carbide forming metal.

By this method it is possible to secure this stoichiometric relation between carbon and carbide formingmetal so as to avoid any excess or lack of carbide forming metal. An excess or a lack of those metals in the melt may have a 3 very disagreeable influence on the properties of the alloy.

It is also possible to form oxides and carbides at the same time or successively by the named measures in one melt.

In its broader aspects the invention con- 4 Standing below is a table showing some bending tests performed with alloys according to Tables 1 and 2 showing a large difierence in toughness in adding the above named additions. All tests are heated 100 days at 1200 C. under the same conditions.

1 It has proved possible after bending 180 completely to press the shanks togethe without rupture.

templates steels of substantially the following composition:

Chromium from about 10-40%, silicon from about 0.2-2.5%, aluminium from about 0.19%, the remainder being at least one metal of the iron group except for elements present as im- 1 purities and fine particles of at least a metal 25 compound selected from the group consisting of oxides and carbides, stable at high temperatures of the order of 1200 C. uniformly dispersed in the alloy as an unsoluble suspension.

The amount of the additions may vary within 30 wide limits according to the composition of the alloy and the conditions of the production of the alloy.

. In practice we have found that it is possible to diminish the brittleness of the alloy, if the alloy. 35

contains on an average at least six particles of oxides or carbides or oxides and carbides, per square inch cross-section in a linear enlargement of 250 times. This amount may be enlarged up toa total amount of 10%.

Below compositions of alloys are mentioned adapting the qualities of the described invention. These alloys were tough after heating 100 days at 1200 C.

Table 1 Proportions given herein are by weight.

20 It is known to stabilize grain boundaries in a steel by the presence of impurities such as oxides, carbides and nitrides. Those impurities are concentrated at the dendridic interfaces. Therefore, only minor amounts of impurities can be present in the alloy. This is not sufiicient for attaining the technical efiect of the present invention.

It must be pointed out by contrast that the carbides and oxides finely divided in the alloys of our invention are not impurities in the known sense. They are present in the alloy in quantities greater than normal impurities. They are added to or formed in the alloy and they are distributed throughout the Whole body of the alloy and not only primarily concentrated at the grain boundaries. The technical effect of our invention depends from the line distribution of those added particles throughout the whole alloy.

It was quite impossible until now to manufacture, for example, wires usable as electric resistance material which maintain the ductility after long heating at high temperatures of alloys of that kind. This eiTect is attained for the first time by our invention.

Other Elements 18.." 0. 1 22 4. 5 0.45 Ce 2a... 0. 35 1 5. 5 0.5 Nb 1.1 Ti. 3a.- 0. 0. 5 30 6. 0 0.25 Ga 2.4 Ta..- 4a... 0. 90 1. 5 5. 0 3.75 V 5a... 0. 1. 4 20 5. 5 2.25 Nb 0.4 Ti...

Remainder. Do.

Do. Do. Do.

comparatively may be mentioned that alloys with the following analysis were brittle after a heating of two hours at the same temperature.

We claim:

A heat resistant ferritic alloy having strongly reduced disposition for brittleness at high tem- The toughness of the alloys was examined by ,e peratures of substantially the following composibending test according DIN DVM 1211. ing to this standard the tests were made in the following manner: Test wires of a diameter of 3 mm. were inserted between two cylindrical clamping pieces and bent so long to and fro on 180 till rupture occurred.

Accordtion: chromium from about 10-40%, silicon from about 0.2-2.5%, aluminium from about 0.1-9%, the remainder being iron except for elements present as impurities and fine particles of a mixture of oxides and carbides in an amount from 76 0,0540% stable at high temperatures of the or- 5 der of 1200 C. uniformly dispersed in the alloy as an insoluble suspension, the alloy containing on an average at least six particles of oxides and carbides per square inch cross-section in a linear enlargement of 250 times.

ERIK H, MATTI'ASSON HAGGLUND.

NILS GiisTA REHNQVIST.

REFERENCE S CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,621,523 Clement Mar. 22, 1927 1,850,953 Armstrong Mar. 22, 1932 1,914,102 Binney Mar. 13, 1933 1,981,719 Comstock Nov. 20, 1934 2,021,159 Tafel Nov. 19, 1935 2,160,670 Oswald May 30, 1939 2,169,007 Romp Aug. 8, 1939 2,174,025 Wise et al Sept. 26, 1939 2,210,309 Swinden Aug. 6, 1940 OTHER REFERENCES Co-operative Bulletin No. 65, Mining and Metallurgical Investigations, pages 5 and 6. Published in 1934 by the Mining and Metallurgical Advisory Boards, Pittsburgh, Pa. 

